1. Field of the Disclosure
The present disclosure relates to a protection method in an optical communication system and an optical communication system capable of realizing the protection method.
2. Discussion of the Background Art
With an increasing demand for high-speed access services, fiber to the home (FTTH) has been spread worldwide. A large part of FTTH services are provided by a passive optical network (PON) system which is economically excellent and in which one optical subscriber unit (OSU) accommodates a plurality of optical network units (ONUs) by time division multiplexing (TDM). In upstream communication of TDM-PON, a system bandwidth is shared between ONUs based on dynamic bandwidth allocation calculated in the OSU, and as illustrated in FIG. 1, each ONU 200 transmits signal light intermittently only in an allowed transmission period notified from an OSU 51 to prevent collision of signal lights. Gigabit Ethernet (registered trademark) PON (GE-PON) and Gigabit-capable PON (G-PON) which have a transmission speed of the Gigabit class are presently the main systems; however, there is a demand for a PON system having a larger capacity with the introduction of applications that enables files of large capacity to be uploaded and downloaded as well as the development of video distribution services. However, since the TDM-PON system extends the system bandwidth by increasing a line rate, the reception characteristics deteriorate dramatically due to the influence of the accelerated speed and the wavelength dispersion, and burst transceivers may cause an economic problem and thus, it is difficult to increase the capacity of the PON system exceeding 10 gigabits.
Application of wavelength division multiplexing (WDM) techniques has been considered to realize a large capacity exceeding 10 gigabits. FIG. 2 illustrates an example of a WDM/TDM-PON in which WDM techniques are combined with TDM-PON. A downstream wavelength and an upstream wavelength are fixedly allocated for each ONU 200a depending on a terminal of a wavelength routing unit 152, to which the ONU will be connected via an optical fiber transmission line, and a temporal overlap of signals between all ONUs is allowed up to the number of OSUs. Thus, it is possible to extend the system bandwidth without increasing the line rate per wavelength by adding an additional OSU.
ONUs connected to the same terminals among the terminals of the wavelength routing unit 152 via the optical fiber transmission line are logically connected to the same OSU and share an upstream bandwidth and a downstream bandwidth. Here, the logical connection between ONUs and OSUs is invariable, it is not possible to distribute a traffic load between different OSUs regardless of a traffic load state of each OSU.
In contrast, Non-Patent Literature 1 proposes a wavelength-tunable WDM/TDM-PON in which an optical transmitter and an optical receiver mounted in an ONU have a wavelength tuning function (FIG. 3). In this configuration, a logical connection destination OSU can be changed for respective ONUs by switching the transmission and reception wavelengths in the ONU. By using this function, when an OSU in a heavy load state is present, the logical connection between ONUs and OSUs can be changed so that a traffic load is distributed to an OSU in a light load state to prevent deterioration in the communication quality in the OSU in the heavy load state. Moreover, when the heavy load state of an OSU occurs normally, the WDM/TDM-PON configuration illustrated in FIG. 2 needs to increase the system bandwidth to secure a constant communication quality, but the wavelength-tunable WDM/TDM-PON configuration illustrated in FIG. 3 can secure a constant communication quality by distributing the traffic load between OSUs to effectively utilize the entire system bandwidth, and thus, it is possible to suppress an additional investment in equipment for increasing the system bandwidth.
In the wavelength-tunable WDM/TDM-PON, the logical connection destination OSU can be changed for respective ONUs as described above. Thus, the wavelength-tunable WDM/TDM-PON enables a protection operation in which when an OSU fails, an ONU logically connected to the failed OSU changes a destination OSU to another OSU to resume communication (FIG. 4 and Non-Patent Literature 2).